1. Field of the Invention
The present invention relates to a method for measuring a pattern formed on a sample and a device for measuring the pattern, and particularly to a method and device for extracting a contour from a pattern edge in an image acquired with a scanning electron microscope or other device.
2. Description of the Related Art
In semiconductor manufacturing processes, a scanning electron microscope (hereinafter sometimes simply referred to as an SEM) or other charged particle beam device is used to measure a line width of a semiconductor device and evaluate the quality thereof. In particular, an SEM for measuring a line width of a line pattern and the diameter of a hole of a hole pattern of a semiconductor device is called a CD-SEM (Critical Dimension SEM).
Conventionally, CD-SEM-based measurement has been used to obtain one-dimensional measurement results, which are pattern dimensions, but two-dimensional geometries have been becoming an important evaluation item as well as one-dimensional dimensions in recent years.
To evaluate a two-dimensional geometry of a pattern, it is necessary to accurately identify an edge of the pattern. An edge portion in an SEM image, however, has a certain width and hence does not suit detailed geometry evaluation. To solve the problem, JP Patent Publication (Kokai) No. 2006-66478 (corresponding US Patent Application No. 2006/0045326) describes a technology for extracting a contour from a pattern edge in an SEM image. JP Patent Publication (Kokai) No. 2006-66478 (corresponding US Patent Application No. 2006/0045326) describes a technology for extracting a contour based on information on the change in brightness of a white band of a pattern edge.
Further, WO 2007/094439 describes a technology for redetermining a once-determined contour with higher precision, in which an SEM's partial scan direction is determined based on the determined contour and then a contour is extracted based on new brightness change information obtained based on the partial scan.
According to the descriptions in JP Patent Publication (Kokai) No. 2006-66478 (corresponding US Patent Application No. 2006/0045326) and WO 2007/094439, two-dimensional geometry evaluation can be carried out based on an accurately extracted contour of a pattern, but they do not describe evaluation of inclination of an edge portion of a pattern. For example, a photomask, which is one of the objects measured with an SEM, has a light-blocking pattern formed on a quartz substrate, the light-blocking pattern formed of a chromium film and corresponding to a circuit design pattern. A thus formed photomask is used in a semiconductor light-exposure device to transfer the design pattern onto a wafer in a manner similar to how a negative photographic film works.
A light-blocking pattern on a photomask tends to decrease in thickness as semiconductor devices have been miniaturized in recent years, but still has a certain thickness. When an edge portion of a light-blocking pattern having a certain thickness is inclined, the result of light exposure differs from that obtained from a vertical edge portion in some cases.
In particular, when an edge is tapered, the light from an optical exposure device is probably incident obliquely, possibly resulting in a smaller post-exposure pattern than expected. Evaluation of inclination of an edge portion can therefore be important in photomask geometry evaluation. Evaluation of the geometry of an edge portion is probably considered important in evaluating the quality of a pattern formed on a wafer as well.
A description will be made of a sample measuring method and a sample measuring device suitable for evaluation of inclination of a pattern edge.